We propose, in this project, the hypothesis that the alkaline phosphatase of the brush border membrane of the renal proximal tubule is involved in the inorganic phosphorus transport. A topographical study of both phenomena, presence of alkaline phosphatase and phosphorus reabsorption, has already been confirmed. We also found in vitamin D-resistant hypophosphatemic mice a relatively low tubular alkaline phosphatase accompanying the Pi transport defect. We intend during the next year to further investigate the correlation between Pi transport and alkaline phosphatase activity in various physiological situations.